EAST 501 Lecture Notes - Lecture 10: Type Ii Collagen, Mesenchymal Stem Cell, Type I Collagen
21 Jun 2018
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10- M. Murshed
15- Skeletal development (1)
1- Bone composition, types and development
●Tooth
○At least four tissues are mineralized. Have hard tissues and soft tissues.
■Bone is the largest hard tissue.
■Enamel, Dentin, and Cementum. Enamel is very different from the others: it’s the
hardest mineralized tissue in our body (no inorganic matrix, only organic ECM).
●Bone
○Most abundant mineralized tissue in the body.
○Provides support to the body, protects the internal soft organs, helps in locomotion.
○Serves as a reservoir for essential minerals (calcium and inorganic phosphate).
■Absorbed from the gut.
■Used for many different physiological functions.
■Have to maintain particular levels of these minerals. To compensate for any
lacking, need to mobilize calcium from bone.
○Functions as an endocrine organ.
■These cells secrete soluble factors, which are released into the circulation and can
act on organs (kidney, pancreas) to regulate function.
○Very high rate of tissue remodeling.
■Bone remodeling → can change the skeleton every ten years.
○Complex tissue architecture and development.
●Bone components
○Cells, matrix, and minerals.
■Cells: primarily of two types → mesenchymal stem cells and hematopoietic
stem cells.
●From mesenchymal stem cells, get chondrocytes in the cartilage, and
osteoblasts in the bone. Osteoblasts can become osteocytes.
●Osteoblasts form an ECM scaffold to facilitate mineralization. When
they stop producing new bone, bury themselves within the matrix they
synthesized, and become osteocytes. Extend out processes. Work more
as regulators.
●Chondrocytes produce cartilage. Some can actually trans differentiate to
osteoblasts.
●Osteocytes are bone resorbing. If need to mobilize minerals or repair
bone → need bone resorption.
■Matrix:
●Collagen forms a network. Most important for mineralization.
○Collagen type I.
●Non-collagenous proteins like fibronectin.
●Mucopolysaccharides (carbohydrates).
○Hydrated. Can be reservoir for signaling molecules.
■Minerals: calcium, phosphorus.
●Bone types
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10- M. Murshed
○Shapes: long bones (limbs), short bones (toes), flat bones (skull), irregular bones (nose).
○Morphology: cortical bone (= compact bone → outer layer of the bone), cancellous bone
(= spongy = trabecular bone → on the inside).
■Beneficial to have spongy bones: shock absorbing (better load-bearing capacity)
+ lighter.
○Development: intramembranous, endochondral.
■Picture: red tissue is mineralized tissue, and blue tissue is cartilage.
●Intramembranous: skull cap (flat bone).
●Endochondral: rib cage, tibia (long bones).
■Difference: endochondral bones need a cartilage precursor that is eventually
replaced by bone. Intramembranous bones don’t need any cartilage precursors.
●Endochondral bone formation
○Chondrocytes produce type II collagen (only alpha 1 chains form the triple helix).
■Forms a cylindrical structure.
○At the core of this cylinder, cells will grow in size and differentiate into pre
hypertrophic chondrocytes.
■Still make type II collagen, bu also make other proteins like proteoglycans.
○Have the resting zone, proliferating zone, and pre hypertrophic zone.
■Hypertrophic cells are at the core.
■These are different because they make a different kind of collagen. Will start
making more type 10 collagen, which becomes mineralized.
■Type I collagen made by osteoblasts also becomes mineralized. Osteoblasts come
from the bone collar and differentiated chondrocytes.
■These cells then die by apoptosis.
○From the blood, get precursor cells for osteoclasts which populate this site.
■These cells differentiate into osteoclasts (multinucleated cells).
○This process continues for growth of the long bones.
○See summary slide.
●Intramembranous bone formation
○Flat bone formation.
○Start with loosely arranged MSCs which proliferate and condense, and differentiate to
osteoblasts.
■Start making type I collagen.
■Eventually, that collagen will be mineralized.
■Cells at the center have lots of collagen surrounding them. When the matrix
mineralizes → trapped. These cells live in the lacuna and extend cell processes
→ osteocytes.
■HA mineral = hydroxyapatite → crystalline structure (probably initially
amorphous, then become crystalline).
○No need for chondrocytes or cartilage here.
○See summary slide.
●True or false?
○All major cell types in bone differentiate from a single type of stem cells.
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10- M. Murshed
■False.
○Osteoblasts deposit type I collagen.
■True.
○Chondrocytes deposit type II and type X collagen.
■True. Type II collagen is the general marker for chondrocytes, and when they
become hypertrophic, they make type I collagen.
○Development of intramembranous bone requires chondrogenesis.
■False- no involvement of chondrocytes there.
○In the developing skeleton, osteoclasts first resorb mineralized cartilage, not bone.
■True.
○Some hypertrophic chondrocytes transdifferentiate to osteoblasts while others die.
■True.
2- Bone cells, their origin and functions
●Bone
○In bone, primarily osteoblasts and osteoclasts.
■From osteoblasts, get osteocytes.
○In cartilage, have chondrocytes.
●Cell differentiation
○Have on top, resting chondrocytes, proliferating chondrocytes, hypertrophic
chondrocytes, hypertrophic chondrocytes, and minerals.
○Controlled by PTH-related peptide, present at the top.
■Inhibits differentiation of resting chondrocytes.
■Gradient of PTHrp along the long bone.
■Further down, cells can differentiate.
○Signalling molecules, in particular combinations, act on cells to direct differentiation (by
the downregulation or upregulation of particular TFs).
■In the developing embryo, cells experience different amounts of these soluble
factors.
●Transcription factors regulating chondrocyte and osteoblast differentiation
○From MSCs chondro/osteogenic bi-potential cells:
■Regulated by RUNX2, and SOX9.
■RUNX2 is known as the master regulator of skeletal development.
○To chondrocytes:
■Also regulated by RUNX2 and SOX9.
○To pre hypertrophic chondrocytes:
■Need downregulation of SOX9. If not downregulated, won’t have this transition.
○To hypertrophic chondrocytes:
■Need upregulation of SOX9.
○From the chondro/osteogenic bi-potential cells, can get osteoblasts via RUNX2 and
Osterix upregulation.
○From hypertrophic chondrocytes, can also get osteoblasts via Osterix/ RUNX2
upregulation.
●Phenotype of RUNX2-deficient mice
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Document Summary
Enamel is very different from the others: it"s the hardest mineralized tissue in our body (no inorganic matrix, only organic ecm). Most abundant mineralized tissue in the body. Provides support to the body, protects the internal soft organs, helps in locomotion. Serves as a reservoir for essential minerals (calcium and inorganic phosphate). Have to maintain particular levels of these minerals. To compensate for any lacking, need to mobilize calcium from bone. These cells secrete soluble factors, which are released into the circulation and can act on organs (kidney, pancreas) to regulate function. Bone remodeling can change the skeleton every ten years. Cells: primarily of two types mesenchymal stem cells and hematopoietic. From mesenchymal stem cells, get chondrocytes in the cartilage, and stem cells. osteoblasts in the bone. Osteoblasts form an ecm scaffold to facilitate mineralization. When they stop producing new bone, bury themselves within the matrix they synthesized, and become osteocytes. Osteocytes are bone resorbing. bone need bone resorption.
